The current information-oriented society is supported by semiconductor elements typified by a complementary metal-oxide semiconductor (CMOS) based on silicon. Hitherto, the silicon semiconductor industry has achieved miniaturization by continuously restricting a range of sizes to which a fine processing technology such as a lithography technology, an etching technology, or a film formation technology is applied from micrometers to several tens of nanometers to realize high integration and high performance concurrently.
In recent years, a thickness of a silicon layer serving as a semiconductor channel has been reaching an atomic layer level, and material and physical limits thereof have been pointed out.
Graphene is a two-dimensional atomic layer thin film having an extremely small thickness and is also stable chemically and thermodynamically. Graphene is a novel semiconductor material having a high potential to meet the demand. Excellent physical properties of graphene may be utilized to realize a new element excelling the existing elements in performance.
Graphene is obtained by taking out only one layer of graphite, which is a layered substance formed of only sp2-hybridized carbon, and is a monatomic layer planar substance which is very robust against a chemical reaction and is also stable thermodynamically as described above.
Graphene has a structure of a honeycomb-like (pseudo-)two-dimensional sheet in which six-carbon rings each having a regular hexagon shape with a carbon atom being positioned at an apex are arranged without any gap, and has a carbon-carbon distance of about 1.42 angstroms (0.142 nm), and a layer thickness of 3.3 to 3.4 angstroms (0.33 to 0.34 nm) in the case where an underlying base is graphite and about 10 angstroms (1 nm) in the case where an underlying base is a substrate other than graphite.
Regarding a size of a graphene plane, various sizes from a molecular size of a nanometer order to theoretically an infinite size can be assumed as a length of one piece of graphene. In general, graphene refers to one layer of graphite, but includes two or more layers thereof in many cases.
In this case, graphene including one layer, graphene including two layers, and graphene including three layers are called monolayer graphene, bilayer graphene, and trilayer graphene, respectively, and graphenes including up to about 10 layers are collectively called few-layer graphene. Further, graphene other than the monolayer graphene is expressed as multi-layer graphene.
An electronic state of graphene can be described by a Dirac equation in a low-energy region (see Non Patent Literature 1). This point presents a marked contrast to electronic states of substances other than graphene, which can be well described by a Schrödinger equation.
Electron energy of graphene has a linear dispersion relation with respect to a wave number in the vicinity of a K-point, and more specifically, can be expressed by two straight lines having positive and negative slopes corresponding to a conduction band and a valence band. A point at which the two straight lines cross each other is called a Dirac point, where graphene electrons have peculiar electronic physical properties of behaving as fermions having an effective mass of zero. For this reason, graphene exhibits a mobility of 107 cm2V−1s−1 or more, which is the highest value among the existing substances, and has a feature of small temperature dependence.
The monolayer graphene is basically a metal or a semimetal having a band gap of zero. However, when a size of the monolayer graphene is of a nanometer order, the band gap becomes wide, and the monolayer graphene becomes a semiconductor having a finite band gap, depending on a width and end structure of graphene. Further, the bilayer graphene has a band gap of zero in the absence of perturbation. However, when such perturbation as to break the minor symmetry between two graphenes, for example, an electric field is applied, the bilayer graphene is to have a finite gap in accordance with a magnitude of the electric field.
For example, the gap is opened by about 0.25 eV at an electric field of 3 Vnm−1. The trilayer graphene exhibits a semimetallic electronic physical properties in which a conduction band and a valence band overlap each other with a width of about 30 meV. The property in which the conduction band and the valence band overlap each other is close to that of bulk graphite. Graphenes including four or more layers also exhibit semimetallic physical properties, and as the number of layers increases, the electronic physical properties of the graphenes get closer to those of the bulk graphite gradually.
Further, graphene is also excellent in mechanical characteristics, and a Young's modulus of one layer of the graphene is 2 TPa, which is remarkably large. A tensile strength thereof is at the highest level of the existing substances.
In addition, graphene has unique optical characteristics. For example, in a wide electromagnetic wave region from an ultraviolet light region (wavelength: about 200 nm) to a region in the vicinity of terahertz light (wavelength: about 300 μm), a transmittance of graphene is 1−nα (n: number of layers of graphene, n=about 1 to 10, α: fine structure constant, α=e2/2hc∈0=0.0229253012, e: quantum of electricity, h: Planck's constant, ∈0: dielectric constant of vacuum). Thus, the transmittance of graphene is represented only by a basic physical constant instead of a material constant of graphene. This is a unique feature of graphene which cannot be found in the other substance materials.
Further, the transmittance and reflectance of graphene exhibit carrier density dependence in a terahertz light region. This means that the optical characteristics of graphene can be controlled based on an electric field effect. It is also known that the other two-dimensional atomic layer thin films have peculiar physical properties based on the dimensionality.
Graphene has exceptional electronic physical properties and optical characteristics and excellent mechanical characteristics and chemical characteristics as described above. Hence, it is expected that graphene is used in a wide range of industrial fields from chemistry to electronics. In particular, in semiconductor devices and micro mechanical devices in the next-generation electronics, spintronics, optoelectronics, micro- and nanomechanics, and bioelectronics fields, graphene is being deployed throughout the world. Research and development are also being conducted actively with respect to the other two-dimensional atomic layer thin films for the purpose of industrial utilization in the same way as in graphene.
When graphene is used in a semiconductor device such as a channel of a field effect transistor (FET), a substrate for supporting graphene (graphene substrate) is required.
As manufacturing methods for a graphene substrate as conventional technologies, the following four methods have been used.
That is, the four methods are: (1) a manufacturing method using peeling (see Non Patent Literature 2); (2) a manufacturing method using CVD (Chemical Vapor Deposition) (see Non Patent Literature 3); (3) a manufacturing method using pyrolysis of silicon carbide (SiC) (see Non Patent Literature 4); and (4) a manufacturing method using gallium-amorphous carbon interface growth (see Patent Literature 1 and Non Patent Literature 5).
The manufacturing method using peeling according to the above-mentioned item (1) is a method involving peeling graphite-graphene flakes from a graphite crystal such as natural graphite or highly oriented pyrolytic graphite (HOPG) with an adhesive tape, and attaching the flakes to a substrate.
Further, the manufacturing method using CVD according to the above-mentioned item (2) is a method involving subjecting a hydrocarbon such as methane to thermal or plasma decomposition on a substrate on which a metallic catalyst has been deposited from the vapor or a foil serving as a metallic catalyst to grow graphene, removing the unnecessary metallic catalyst with an acid or the like, and transferring the graphene onto another substrate.
Further, the manufacturing method using pyrolysis of silicon carbide (SiC) according to the above-mentioned item (3) is a method involving subjecting a SiC substrate to heat treatment at high temperature (about 1,600° C.), and allowing carbon atoms to aggregate on the substrate while evaporating silicon atoms from the surface of the substrate to grow graphene on the substrate.
Further, the manufacturing method using gallium-amorphous carbon interface growth according to the above-mentioned item (4) is a method involving bringing a liquid gallium layer into contact with an amorphous carbon layer deposited from the vapor on a substrate from above at high temperature (about 1,000° C.) to grow graphene on the amorphous carbon by an interface reaction, transferring the composite layer of gallium layer/graphene layer/amorphous carbon layer to another substrate, and dissolving the gallium with an acid to obtain a composite layer of graphene layer/amorphous carbon layer. According to the method according to the above-mentioned item (4), the amorphous carbon layer/graphene layer/substrate is formed in the stated order with one transfer, and hence graphene is set to be an uppermost layer in response to the demand in terms of manufacturing of a device. That is, in order to obtain the graphene layer/amorphous layer/substrate formed in the stated order, still another transfer (total two transfers) is required.